Thermal barrier coatings (TBCS) are widely used to reduce the operating temperatures of underlying substrates. For example, TBCs have been used for years in gas turbine engines, and more particularly in the turbine sections of such engines.
A typical TBC system utilizes a superalloy substrate, with a thin adherent alumina layer formed over the substrate, and a ceramic layer applied on the alumina layer. See, e.g., U.S. Pat. No. 4,321,311 to Strangman. Depending upon the particular superalloy, a separate bond coat, including but not limited to an MCrAlY or aluminide bond coat is provided on the substrate, and the adherent alumina layer is subsequently formed on the bond coat. M is selected from the group including nickel, cobalt, iron and combinations thereof. Alternatively, some superalloys can be oxidized to form an adherent alumina layer, and no separate bond coat is required. Exemplary alloys are described in commonly-owned U.S. Pat. Nos. 4,209,348 and 4,719,080 both to Duhl et al. A primary benefit of such superalloys is that there is no need to cover the substrate with a separate bond coat. The addition of a bond coat adds weight to a component without adding strength, which while undesirable generally, e.g., in gas turbine engines, is particularly undesirable on moving or rotating parts such as blades. On parts rotating at several thousands of revolutions per minute, the additional weight of the bond coat adds significantly to blade pull, e.g., corresponds to the centrifugal force due to the bond coat and increases with the square of the rotational speed. At elevated temperatures, the blade pull attributable to the bond coat also contributes to creep at the blade root, which affects the clearance between the blade tip and any surrounding structure and also affects engine efficiency and longevity. Moreover, a thick bond coat is subject to significant thermal fatigue due to the thermal stresses generated in the coat over the wide range of temperatures to which the component is exposed. Accordingly, use of superalloys capable of forming an adherent alumina layer are increasingly desired for use in rotating components such as turbine blades and compressor blade, as well as other moving components.
It is known that many ceramic materials, including stabilized or strengthened zirconia generally and by way of example zirconia having 7 percent by weight yttria (7YSZ) described in commonly-owned U.S. Pat. No. 4,321,311 to Strangman, are relatively transparent to oxygen. Accordingly, underlying metal will oxidize (at generally manageable and predicable rates), and will oxidize at an increasing rate as the temperature increases. It is also known that the ceramic layer will eventually spall or otherwise fail, which in turn influences the service life of the component. Under normal operating conditions, service life subsequent to ceramic spallation is affected by the remaining bond coat or alloy oxidation life. As a general rule, the superalloys capable of forming an alumina layer without the use of a separate bond coat tend to be less oxidation resistant than conventional superalloys which utilize a separate bond coat, and we believe that higher oxidation resistance of conventional superalloys is due at least in part to a higher aluminum content, e.g., in the bond coat used with the conventional superalloys, as well as the presence of an intervening layer (the bond coat) between the substrate and its environment.
It is further known that portions of the ceramic material occasionally fail prematurely, for example due to localized spallation or foreign object damage, e.g., particulates formed during combustion, debris entrained in air ingested by the engine, or debris generated by broken upstage components. Underlying, exposed component areas are then subjected to significantly increased temperatures, and oxidize at correspondingly higher rates thereby reducing the life of the component. With respect to components that do not include a separate bond coat, the substrate material is exposed directly to the higher temperatures and increased oxygen, and oxidizes at even higher rates. The higher oxidation rate occurring on unprotected portions of substrate material in turn accelerates failure of the surrounding ceramic and exposure of additional substrate material, and the increased temperatures can melt or otherwise damage the substrate material.
It is an object of the present invention to provide a TBC system, preferably but not necessarily incorporating a superalloy that forms an adherent alumina layer, providing the benefit of reduced weight while still limiting oxidation in the event that the ceramic fails.
It is another object of the invention to provide such a system in which the service life of an associated component is not significantly shortened in the event of ceramic failure.